It is a common practice to blacken the surfaces of the color-selection electrode and its support frame, which together comprises the color-selection electrode assembly, in order to increase the radiative characteristics and to reduce the reflectivity thereof. The term color-selection electrode includes not only the conventional shadow mask and tension mask but also similar structures, adjacent to the luminescent screen of the CRT, which intercept at least a portion of an electron beam directed toward the screen to assure excitation of the proper color-emissive phosphor.
One method of blackening the surfaces has been to fire the color-selection electrode, the frame, or the completed assembly in an oxidizing atmosphere to yield a black oxide of iron. However, the quality of such oxide coatings vary considerably. Also, iron oxide has a tendency to flake off the surfaces and to decompose in vacuum when it is heated and bombarded with electrons, as is the case during operation of the color cathode-ray tube. Additionally, such a conventional process is unsatisfactory for darkening color selection electrodes made from materials having a high nickel content. Also, the elevated temperature required during the firing process tends to non-uniformly affect the tension of some of the elements of the tension mask, rendering some of the masks unacceptable for their intended purpose. A need therefore exists for a process that does not require elevated temperatures, is inexpensive to perform, is applicable to both shadow masks and tension masks, and provides uniform results on steel and low expansion nickel-iron alloys.